Various films for use in the food and health care packaging industries are known. These films are generally selected on the basis of key properties desirable for packaging applications. Packaging films desirably have good mechanical properties to provide strength and processability, good optical clarity, and good barrier properties. In food packaging, this latter property is of extreme importance to maintain the food in a fresh and unspoiled condition by inhibiting the transmission of oxygen and moisture into the package. Other important properties for food packaging films include heat sealability and heat stability. Heat stability is particularly important in those applications wherein the package and the contents thereof are heated together, for example, in boiling water or by microwaves.
Various polymers used in food packaging films generally contain one or more of the desired properties, but not usually all. For example, ethylene-vinyl alcohol copolymer films have good oxygen barrier properties; however, such films do not usually have good heat sealability properties and the oxygen barrier properties thereof are adversely affected by moisture and high humidity. On the other hand, polypropylene has an unacceptably high oxygen permeability, but is heat sealable, heat stable, and as a consequence of its hydrophobicity, it is a good moisture barrier. Therefore, multilayer food packaging films are manufactured by using ethylene-vinyl alcohol copolymer as an interlayer between outer layers of polypropylene. Since polypropylene does not adhere well to ethylene-vinyl alcohol copolymer, it is necessary to employ a tie layer between the layers. Maleated polypropylene is commonly used for this purpose in multiple layer films employed in the food packaging industry. The outer skin layers of polypropylene function as moisture barriers and provide for good heat sealability, heat stability, and structural properties of the package, while the ethylene-vinyl alcohol copolymer interlayer functions as an effective oxygen barrier to preserve the freshness of the food packaged therein. Such films are, however, disadvantageous in that multiple layer films are inherently more complicated and expensive to manufacture than monolayer films.
It is known from EPO publication 207,440 to form a laminated film containing an interlayer of ethylene-vinyl alcohol-vinylpyrrolidone terpolymer, tie layers on opposite sides thereof containing maleated polypropylene, and outer layers containing polypropylene. The film is stated to have good clarity without haze, no cracks, even stretching, and low oxygen permeability.
It is known from U.S. Pat. Nos. 4,451,599 and 4,552,801 to use homopolyvinylpyrrolidone as a plasticizer in a blend with ethylene-vinyl alcohol copolymer to improve elongation, tear strength and impact. This blend can be used in a layer of a laminate containing outer layers such as polyamide or polyolefin which are stated to function as water vapor barriers. An adhesive layer is used between the interlayer and the outer layers.
It is known from U.S. Pat. No. 3,700,449 to use homopolyvinylpyrrolidone as an oxygen-impermeable barrier layer for a photographic photoconductor material.
It is known from U.S. Pat. No. 4,451,260 and Japanese Patent Application 47-20327 to use polyvinylpyrrolidones in pharmaceutical coatings.
It has been suggested to graft vinylpyrrolidone onto polyolefins by ionic irradiation of a mixture of polyolefin and vinylpyrrolidone, by heat polymerization of such a mixture in the presence of a radical polymerization initiator such as a peroxide or azo compound, or by peroxidizing or hydroperoxidizing the polyolefin in advance with an oxidizing agent or to form free radicals of the polyolefin by heat or by a mechanical operation and then contacting the vinylpyrrolidone monomer therewith to carry out the graft copolymerization. In the prior art, this is done in a suspension or solution as described, for example, in U.S. Pat. Nos. 3,458,597; 3,497,573; 3,644,581; 3,644,582; 3,683,049; 3,728,417; 3,752,868, 3,800,007; EPO Publication 1,313; and U.K. Patent Applications 1,114,338, 1,558,991; and 2,022,597; or by grafting the vinylpyrrolidone onto the surface of a solid polyolefin substrate such as a membrane, film, fiber, tube or the like as described, for example, in U.S. Pat. Nos. 3,049,507; 3,073,667; 3,157,560; 3,290,207; 4,743,258; 4,143,218, U.K. Patent Applications 1,338,810; and 1,588,065; and EPO Publications 203,459; and 249,513.
As far as applicant is aware, there is no prior art teaching the grafting of a vinylpyrrolidone onto a polyolefin by melt blending the polyolefin at elevated temperatures with vinylpyrrolidone in the presence of a free radical initiator. Nor is applicant aware of any prior art teaching the use of a vinylpyrrolidone grafted polyolefin in blends of incompatible polar and non-polar polymers, e.g. blends of ethylene-vinyl alcohol copolymer and polypropylene, for the purpose of compatibilizing the incompatible blend components, nor the use of a vinylpyrrolidone grafted polyolefin in a blend with an oxygen barrier resin such as ethylene-vinyl alcohol copolymer for the purpose of promoting adhesion to a non-polar layer such as polypropylene or polyethylene in a tieless multilayer film, or blended with polypropylene to promote adhesion to ethylene-vinyl alcohol copolymer. Finally, applicant is not aware of any prior art teaching a low oxygen permeable monolayer film containing a hydrophobic water barrier resin such as polypropylene, a hydrophilic oxygen barrier resin such as ethylene-vinyl alcohol copolymer and a vinylpyrrolidone grafted polyolefin.